Leadscrew mechanical drive with differential leadscrew follower structure and brake

ABSTRACT

A leadscrew mechanical drive includes a leadscrew having a leadscrew axis and a leadscrew thread with a nonzero leadscrew thread pitch. A leadscrew follower structure is engaged to the leadscrew. The leadscrew follower structure includes a first leadscrew follower and, desirably, a second leadscrew follower. The first leadscrew follower and the second leadscrew follower are substantially identical, and each leadscrew follower includes a threaded insert having an insert thread with a nonzero insert thread pitch different from the leadscrew thread pitch. The insert thread is engaged to the leadscrew thread over a circumferential distance of less than one-half turn of the insert thread. Each leadscrew follower further includes a bearing in which the threaded insert is received and which permits the threaded insert to rotate about an insert axis that is inclined to the leadscrew axis by an angle θ for the first leadscrew follower and −θ for the second leadscrew follower, and a bearing support in which the bearing is receive. A preload structure biases each of the leadscrew followers against the leadscrew. A brake is selectively engagable to the threaded inserts to selectively permit the threaded inserts to rotate about the insert axis or prevent the threaded inserts from rotating about the insert axis.

This application is a continuation-in-part of application Ser. No.10/271,479, filed Oct. 15, 2002, now U.S. Pat. No. 6,931,960, for whichpriority is claimed and whose disclosure is incorporated by reference.

This invention relates to a leadscrew mechanical drive and, moreparticularly, to a leadscrew mechanical drive utilizing a leadscrew anda leadscrew follower structure with selectively controllable coarse andfine advancement settings.

BACKGROUND OF THE INVENTION

Most common motors have a rotational output, but in many instances thestructure to be driven requires a linear motion. There are a number ofapproaches to mechanically converting the rotational motor output to alinear motion. One such approach is a leadscrew mechanical drive inwhich the motor rotationally drives a threaded leadscrew. A leadscrewfollower structure driven by the leadscrew engages the leadscrew andmoves parallel to the axis of the leadscrew to convert the rotationalmotion to linear motion parallel to the axis of the leadscrew. Theleadscrew follower structure typically includes a follower threadablyengaged to the leadscrew or a recirculating ball mechanism, althoughother types of leadscrew followers have been proposed.

The standard leadscrew mechanical drive works well in many situations,but has limitations in others. If the linear motion is to be very slow,the basic leadscrew mechanical drive must be modified. The leadscrewmust have a small thread pitch, the leadscrew must be driven through aspeed-reducing gearbox placed between the motor and the leadscrew,and/or a special motor control such as a high-resolution rotary opticalencoder must be used to measure and control the motor output in afeedback manner. Additionally, care must be taken to minimize play andbacklash in the leadscrew mechanical drive. Some of these modificationshave physical limitations that limit the ability to achieve the desiredslow linear movement. For example, the reduction in thread pitch islimited by the physical width between the turns of the thread that maybe achieved by machining or other thread-producing technique. Theability to reduce the rotational output speed of the motor is limited bythe structure of the motor windings, so that it may be necessary to usea special low-speed motor. In any event, these modifications lead to anexpensive, bulky, and/or heavy leadscrew mechanical drive that may notbe suitable for the desired applications. Additionally, the coarsepositioning movement of the leadscrew follower structure to its basicslow-movement operating position for large distances along the length ofthe leadscrew is very slow.

There is therefore a need for an improved approach to the mechanicalconversion of rotary motion to linear motion, particularly forapplications in which the required linear motion is very slow. Thepresent invention fulfills this need, and further provides relatedadvantages.

SUMMARY OF THE INVENTION

The present invention provides a leadscrew mechanical drive with acontrollable transmission ratio between the rotational rate of theleadscrew and the linear rate of motion of the leadscrew followerstructure. The linear motion of the leadscrew follower structure may beselected to be faster or slower than that expected from the normalleadscrew mechanical drive, but the greatest advantage is achieved whenthe linear motion is slower than that expected from the normal leadscrewmechanical drive. In that instance, the speed reduction is accomplishedentirely in the leadscrew/leadscrew follower mechanism. No modificationto the motor or motor controller is required, and no separate gearbox isused. Additionally, the present approach allows the leadscrew followerstructure to be operated in either of two rates of movement, a fast rateor a slow rate. For example, the leadscrew follower structure may bemoved for relatively large distances over the length of the leadscrew toits desired operating position rapidly at a “coarse” selective setting,and then driven very slowly at that position with a “fine” selectivesetting.

The leadscrew mechanical drive is inexpensive, light, and compact. If acontrolled-resolution motor such as a stepper motor is used to power theleadscrew mechanical drive, high linear positional resolution andaccuracy, coupled with a low movement rate, may be readily achieved inthe fine setting. The leadscrew itself may be manufactured with areadily controlled small thread pitch, so that no special manufacturingprocedures are required for the leadscrew. The present leadscrewmechanical drive is most suitable for relatively light dutyapplications, where the rate of linear motion is relatively slow and theforces to be transmitted are relatively small.

In accordance with the invention, a leadscrew mechanical drive comprisesa leadscrew having a leadscrew axis and a leadscrew thread with anonzero leadscrew thread pitch, and a leadscrew follower structureengaged to the leadscrew. The leadscrew may be of any type, such as aconventional machined leadscrew or a wire-wound leadscrew. The leadscrewfollower structure includes a threaded insert having an insert threadwith a nonzero insert thread pitch different from the leadscrew threadpitch. The insert thread is engaged to the leadscrew over acircumferential distance of less than one-half turn (preferably lessthan about 10 degrees, and most preferably a line contact) of the insertthread. The leadscrew follower structure further includes a bearing inwhich the threaded insert is received and which permits the threadedinsert to rotate about an insert axis that is inclined to the leadscrewaxis, and a bearing support in which the bearing is received.Preferably, the insert axis is inclined to the leadscrew axis by anangle that is greater than zero and less than about 45 degrees, and morepreferably less than about 32 degrees.

A brake is selectively engagable to the threaded insert to selectivelypermit the threaded insert to rotate about the insert axis or preventthe threaded insert from rotating about the insert axis. When the brakeis not engaged and the threaded insert rotates about the insert axis,the leadscrew follower structure moves very slowly—its fine setting.When the brake is engaged and the threaded insert does not rotate aboutthe insert axis, the leadscrew follower structure moves rapidly, itscoarse setting.

There is desirably a preload forcing the insert thread against theleadscrew thread to minimize play in the drive and backlash when thedirection of the drive is reversed.

The leadscrew thread pitch may be greater or less than the insert threadpitch, but the greatest advantage is achieved in a speed reducing drivein which the leadscrew thread pitch is greater than the insert threadpitch. The insert thread lay may be the same as or different than theleadscrew thread lay. As used herein, the “thread lay” is either aleft-hand or a right-hand thread.

The benefits of the present approach may be achieved using a singleleadscrew follower, but a preferred design uses two cooperating andbalanced leadscrew followers. Thus, a leadscrew mechanical drivecomprises a leadscrew having a leadscrew axis and a leadscrew threadwith a nonzero leadscrew thread pitch, and a leadscrew followerstructure engaged to the leadscrew. The leadscrew follower structureincludes a first leadscrew follower and a second leadscrew follower. Thefirst leadscrew follower and the second leadscrew follower aresubstantially identical in structure. Each leadscrew follower includes athreaded insert having an insert thread with a nonzero insert threadpitch different from the leadscrew thread pitch. The insert thread isengaged to the leadscrew over a circumferential distance of less thanone-half turn of the insert thread. The leadscrew follower structurefurther includes a bearing in which the threaded insert is received andwhich permits the threaded insert to rotate about an insert axis that isinclined to the leadscrew axis by an inclination angle for the firstleadscrew follower and a negative of the inclination angle for thesecond leadscrew follower. A preload structure biases each of theleadscrew followers against the leadscrew. A wedge brake is selectivelyengagable to the threaded inserts to selectively permit the threadedinserts to rotate about their respective insert axis or prevent thethreaded inserts from rotating about their respective insert axis.Compatible features discussed elsewhere herein may be used with thisembodiment.

In this preferred form of the leadscrew follower structure using twobalanced leadscrew followers, the preload structure comprises afirst-leadscrew follower pivot about which the first leadscrew followerpivots, and a second-leadscrew follower pivot about which the secondleadscrew follower pivots. The first-leadscrew follower pivot and thesecond-leadscrew follower pivot are adjacent to each other. A preloadsuch as a spring forces the two leadscrew followers to pivot away fromeach other about their respective pivots to produce an engagingfrictional contact between the threads of the leadscrew and theleadscrew followers.

More generally, a leadscrew mechanical drive comprises a leadscrewhaving a leadscrew axis and a leadscrew thread with a nonzero leadscrewthread pitch, and a leadscrew follower structure engaged to theleadscrew, wherein the rate of advance of the leadscrew followerstructure relative to a rate of turning of the leadscrew about theleadscrew axis is selectively controllable between two different ratesof advance. In the preferred structure, the selective controllability isachieved by operating the brake in the manner discussed above.

In a conventional leadscrew mechanical drive where the thread pitch ofthe leadscrew and the thread pitch of the leadscrew follower structureare the same, the leadscrew follower structure engages the leadscrewover several turns and moves linearly by the amount of the thread pitchwith each revolution of the leadscrew. In the present approach where thethread pitches of the leadscrew and the leadscrew follower structure aredifferent, the leadscrew follower structure moves linearly by thedifference between the thread pitches with each revolution of theleadscrew, thereby achieving a differential drive effect. A small linearmovement rate may be achieved by making the thread pitches of theleadscrew and the leadscrew follower structure close to the same butslightly different. Having a different thread pitch of the leadscrew andthe leadscrew follower structure is not possible with conventionalleadscrews, because the mismatch would lead to binding between the twothreads. In this case, however, binding between the thread of theleadscrew and the thread of the leadscrew follower structure is avoidedby inclining the two sets of threads to each other and achieving acontact over a circumferential distance of less than about one-halfturn, and preferably a line contact between the two threads over acircumference of less than about 10 degrees.

In the absence of the brake, the leadscrew follower structure can moveonly very slowly. It is therefore time consuming to move the leadscrewfollower structure for long distances within the available travelpermitted by the length of the leadscrew. With the present approach, theapplication of the brake to the threaded insert stops their rotationalmotion, causing the leadscrew follower structure to move at a ratedetermined by the leadscrew thread pitch and thence at a fast rate for“coarse” setting of the leadscrew follower structure. When the brakingis removed from the threaded insert, the leadscrew follower structuremoves at a rate determined by the difference in the leadscrew threadpitch and the insert thread pitch, which difference may be made verysmall. The result is a slow rate of movement for “fine” setting of theleadscrew follower structure. This controllability of the rate ofmovement of the leadscrew follower assembly between coarse and finesettings provides a great convenience in the use of the leadscrewmechanical drive.

Other features and advantages of the present invention will be apparentfrom the following more detailed description of the preferredembodiment, taken in conjunction with the accompanying drawings, whichillustrate, by way of example, the principles of the invention. Thescope of the invention is not, however, limited to this preferredembodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a leadscrew mechanical drive;

FIG. 2 is a schematic detail of the leadscrew and a preferred form ofthe leadscrew follower structure used in the leadscrew mechanical driveof FIG. 1, with the brake not applied to the threaded insert; and

FIG. 3 is the same schematic detail of the leadscrew and a preferredform of the leadscrew follower structure as in FIG. 2, except with thebrake applied to the threaded insert.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 depicts a leadscrew mechanical drive 20 according to oneembodiment of the present approach. The leadscrew mechanical drive 20includes an externally threaded leadscrew 22 that is driven by arotational output 24 of a motor 26 or other power source. A leadscrewfollower structure 28 is engaged to the leadscrew 22. As the motor 26turns the leadscrew 22 about its rotational leadscrew axis 30, thethreaded engagement between the leadscrew follower structure 28 and theleadscrew 22 translates the rotational movement of the leadscrew 22 intolinear movement of the leadscrew follower structure 28 in a linearmovement direction 32 parallel to the rotational leadscrew axis 30. Theleadscrew follower structure 28 is attached to the structure (not shown)that is to be moved in the linear manner.

FIGS. 2 and 3 schematically depict the leadscrew 22 and the leadscrewfollower structure 28 of a preferred embodiment of the present approachin greater detail. The elements shown in FIGS. 2 and 3 are the same, andthe following descriptions apply to both. The difference in FIGS. 2 and3 is the position of the brake, which will be discussed subsequently.

The leadscrew 22 comprises a helical leadscrew thread 40 having a seriesof turns 42 with a thread pitch TP_(LS). The thread pitch of theleadscrew 22 is defined as the linear distance measured parallel to therotational leadscrew axis 30 between two adjacent turns 42 of theleadscrew thread 40. The thread pitch is nonzero and may be either aleft-hand lay or a right-hand lay. The illustrated leadscrew thread 40is a wire-wound leadscrew of the type disclosed in U.S. Pat. No.5,636,549, whose disclosure is incorporated by reference. The leadscrewthread 40 may instead be a machined thread or any other operable type.

The leadscrew follower structure 28 is engaged to the leadscrew 22. Theleadscrew follower structure 28 includes at least one, and preferablytwo, leadscrew followers in a cooperating relationship. The presentapproach is operable with a single leadscrew follower of the typedescribed next, but the force-balanced approach achieved using twoleadscrew followers is preferred and will be described.

The leadscrew follower structure 28 includes a first leadscrew follower50 and a second leadscrew follower 52. The physical structures of thefirst leadscrew follower 50 and the second leadscrew follower 52 arepreferably substantially identical, and the two leadscrew followers 50and 52 are arranged in a cooperating manner as will be described. Eachleadscrew follower 50, 52 includes a threaded insert 54 having aninternal insert thread 56 with a nonzero insert thread pitch TP_(I). Animportant feature of the present approach is that the insert threadpitch TP_(I) is different from the leadscrew thread pitch TP_(LS). Thepresent approach of a differential leadscrew follower structure is notoperable if the insert thread pitch TP_(I) is the same as the leadscrewthread pitch TP_(LS), or if either of the thread pitches is zero. If thetwo thread pitches TP_(I) and TP_(LS) were the same, the insert 54 wouldidle in one position and not advance in either direction.

The insert thread lay may be the same as a leadscrew thread lay, ordifferent from a leadscrew thread lay. (The “lay” is the sense of thewinding and advance of a helical thread relative to the axis of thehelix, and is usually expressed as a “right-hand lay” or a “left-handlay”.) The present approach may be used where the leadscrew thread 40and the insert thread 56 are either both left-hand or both right-hand,or where one is left-hand and the other is right-hand.

Each of the threaded inserts 54 is received in a bearing 58 whichpermits the threaded insert 54 to rotate about an insert axis 60 that isinclined to the leadscrew axis 30 by an inclination angle θ. The bearing58 may be of any operable type, and is illustrated as a roller bearing.If there are two leadscrew followers 50 and 52 as in the illustratedembodiment, it is preferred that the inclination angle of the insertaxis 60 of the first leadscrew follower 50 is θ, and that theinclination angle of the insert axis 60 of the second leadscrew follower52 is the negative of the inclination angle for the first leadscrewfollower, or −θ. The value of the inclination angle θ, by which theinsert axis 60 is inclined to the leadscrew axis 30, is greater thanzero and less than about 45 degrees, and more preferably less than about32 degrees. Inclination angles of greater than about 45 degrees lead toslippage between the two threads 40 and 56 for all thread profiles.Inclination angles of greater than about 32 degrees lead to slippagebetween the two threads for conventional 30-degree thread profiles. Thisarrangement facilitates the preloading that is discussed subsequently.

The inclination between the insert axis 60 and the leadscrew axis 30causes the insert thread 56 to be engaged to the leadscrew thread 40 ona contact 66 extending over a circumferential distance of less thanone-half turn of the insert thread 56 and one-half turn of the leadscrewthread 40. More preferably, the contact between the insert thread 56 andthe leadscrew thread 40 is a line contact extending a circumferentialdistance of about 10 degrees or less of the insert thread 56 and theleadscrew thread 40. (Although ideally the contact extends zero degreesin the circumferential direction, due to some play and deformation ofthe elements, it may extend a few degrees, but normally less than about10 degrees.) By engaging the two threads 56 and 40 over such a shortcircumferential distance, there is substantially no binding of the twothreads 56 and 40 of different pitches. On the other hand, the shortcontact distance limits the force that may be transmitted through theleadscrew 22 to the leadscrew follower structure 28 to a relativelysmall value to ensure that the threads are not deformed.

The bearing 58 is received in and supported by a bearing support 62. Thebearing support 62 holds the bearing 58 and thence the insert 54 in thedesired orientation with the desired inclination angle(s). The bearingsupport 58 is supported by a follower-structure housing 64.

A preload structure 68 biases each of the leadscrew followers 50 and 52(or a single leadscrew follower, where used) against the leadscrew 22.That is, the insert thread 56 is forced against the leadscrew thread 40to minimize slippage between the two threads, to minimize play in thestructure, and to minimize backlash when the direction of rotation ofthe leadscrew 22 is reversed. Any operable preload structure 68 may beused. In a preferred preload structure 68 illustrated in FIGS. 2 and 3,there is a first-leadscrew follower pivot 70 about which the firstleadscrew follower 50 pivots, and a second-leadscrew follower pivot 72about which the second leadscrew follower 52 pivots. The first-leadscrewfollower pivot 70 and the second-leadscrew follower pivot 72 areadjacent to each other. The pivots 70 and 72 may be dowels extendingparallel to each other from the follower-structure housing 64 andthrough respective bores 74 and 76 in the respective bearing supports62. A preload forces the two leadscrew followers 50 and 52 to pivot awayfrom each other about their respective pivots 70 and 72. The preload isconveniently produced by a preload spring 78, in this case a wire orleaf spring, that is supported upon and reacts against a support pin 80that extends from the follower-structure housing 64 parallel to thepivots 70 and 72, but at the opposite end of the bearing supports 62.The ends of the preload spring 78 react against the ends of the bearingsupports 62 opposite from the respective pivots 70 and 72 to bias thebearing supports 62 outwardly away from the spring support pin 80. Thispreloading of the bearing supports 62 and thence the inserts 54 createsa secure, play-free, backlash-free contact between the leadscrew thread40 and the insert thread 56.

A brake 90 is controllably engagable to the threaded insert toselectively permit the threaded insert 54 to rotate about the insertaxis 60, or prevent the threaded insert 54 from rotating about theinsert axis 60. In the illustrated embodiment having two threadedinserts 54, the brake 90 is structured to operate on both threadedinserts 54 at once. The brake 90 may be of any operable type. In theillustrated preferred embodiment, the brake 90 is formed as a yoke 92having two legs 94. An engagement area 96 at the end of each leg 94 isdimensioned to engage in a wedging action the facing side surface 98 ofthe respective threaded insert 54 when the yoke 92 is moved downwardly,as in FIG. 3, or to be free of contact with the threaded insert 54 whenthe yoke 92 is moved upwardly, as in FIG. 2. The contacting position ofFIG. 3 prevents the threaded inserts 54 from turning about the insertaxis 60, while the non-contacting position of FIG. 2 allows the threadedinserts 54 to turn about the insert axis 60.

The amount of linear advance LA of the leadscrew follower structure 28,relative to the leadscrew thread pitch and the insert thread pitch,depends upon whether the threaded insert 54 is allowed to turn about theinsert axis 60, that is, where the brake 90 is not engaged (FIG. 2); orwhether the threaded insert 54 is not allowed to turn about the insertaxis 60, that is, where the brake 90 is engaged (FIG. 3).

In the case where the threaded insert 54 is allowed to turn about theinsert axis 60 as in FIG. 2, the amount of linear advance LA of theleadscrew follower structure 28 per turn of the leadscrew 22 about itsrotational leadscrew axis 30 (i.e., its rate of advance relative to therate of turning of the leadscrew 22 about the leadscrew axis 30)parallel to the linear movement direction 32, for each revolution of theleadscrew 22 about its rotational leadscrew axis 30, isLA=(TP_(LS)−TP_(I)). By making TP_(LS) and TP_(I) close to each otherbut not the same value, LA may be made arbitrarily small and less thaneither TP_(LS) or TP_(I). Thus, to present a numerical example, ifTP_(I) is 3.0015 millimeters and TP_(LS) is 3.0000 millimeters, thelinear advance LA per revolution of the leadscrew 22 is 3.0015−3.0000millimeters, or 0.0015 millimeters.

In the case where the threaded insert 54 is not allowed to turn aboutthe insert axis 60 as in FIG. 3, the amount of linear advance LA of theleadscrew follower structure 28 parallel to the linear movementdirection 32, for each revolution of the leadscrew 22 about itsrotational leadscrew axis 30, is LA=TP_(LS). Thus, in the abovenumerical example, the linear advance LA per revolution of the leadscrew22 is 3.0000 millimeters.

That is, the rate of linear advance may be altered by a factor ofTP_(LS)/(TP_(LS)−TP_(I)), as between the coarse and fine settings. Inthe numerical example presented in the preceding two paragraphs, thechange in advancement rate of the leadscrew follower between the coarseand fine operating settings is a factor of 2000. The coarse setting withthe brake engaged is suitable for rapid movement of the leadscrewfollower structure 28 to various positions along the length of theleadscrew 22. The fine setting with the brake not engaged is suitablefor very slow movement of the leadscrew follower structure 28 at aparticular operating position along the leadscrew 22 in order to achievefine adjustment of the position of the leadscrew follower structure 28.This capability to switch between the coarse-setting and fine-settingconfigurations is a great convenience in practical operations.

The present invention as illustrated in FIGS. 1–3 has been reduced topractice and found to be operable as described herein.

Although a particular embodiment of the invention has been described indetail for purposes of illustration, various modifications andenhancements may be made without departing from the spirit and scope ofthe invention. Accordingly, the invention is not to be limited except asby the appended claims.

1. A leadscrew mechanical drive comprising: a leadscrew having aleadscrew axis and a leadscrew thread with a nonzero leadscrew threadpitch; a leadscrew follower structure engaged to the leadscrew, theleadscrew follower structure including a threaded insert having aninsert thread with a nonzero insert thread pitch different from theleadscrew thread pitch, wherein the insert thread is engaged to theleadscrew over a circumferential distance of less than one-half turn ofthe insert thread, a bearing in which the threaded insert is receivedand which permits the threaded insert to rotate about an insert axisthat is inclined to the leadscrew axis, and a bearing support in whichthe bearing is received; and a brake that is selectively engagable tothe threaded insert to selectively permit the threaded insert to rotateabout the insert axis or prevent the threaded insert from rotating aboutthe insert axis.
 2. The leadscrew mechanical drive of claim 1, whereinthe leadscrew thread pitch is greater than the insert thread pitch. 3.The leadscrew mechanical drive of claim 1, wherein the leadscrew threadpitch is less than the insert thread pitch.
 4. The leadscrew mechanicaldrive of claim 1, wherein an insert thread lay is the same as aleadscrew thread lay.
 5. The leadscrew mechanical drive of claim 1,wherein an insert thread lay is different from a leadscrew thread lay.6. The leadscrew mechanical drive of claim 1, wherein the insert axis isinclined to the leadscrew axis by an angle that is greater than zero andless than about 45 degrees.
 7. The leadscrew mechanical drive of claim1, wherein the leadscrew comprises a wire-wound leadscrew.
 8. Theleadscrew mechanical drive of claim 1, further including a preloadstructure forcing the insert thread against the leadscrew thread.
 9. Theleadscrew mechanical drive of claim 1, wherein the insert thread isengaged to the leadscrew over a circumferential distance of less thanabout 10 degrees.
 10. A leadscrew mechanical drive comprising: aleadscrew having a leadscrew axis and a leadscrew thread with a nonzeroleadscrew thread pitch; a leadscrew follower structure engaged to theleadscrew, wherein the leadscrew follower structure includes a firstleadscrew follower and a second leadscrew follower, wherein the firstleadscrew follower and the second leadscrew follower are substantiallyidentical, and wherein each leadscrew follower includes a threadedinsert having an insert thread with a nonzero insert thread pitchdifferent from the leadscrew thread pitch, wherein the insert thread isengaged to the leadscrew over a circumferential distance of less thanone-half turn of the insert thread, a bearing in which the threadedinsert is received and which permits the threaded insert to rotate aboutan insert axis that is inclined to the leadscrew axis by an inclinationangle for the first leadscrew follower and a negative of the inclinationangle for the second leadscrew follower, and a bearing support in whichthe bearing is receive; a preload structure that biases each of theleadscrew followers against the leadscrew; and a wedge brake that isselectively engagable to the threaded inserts to selectively permit thethreaded inserts to rotate about the insert axis or prevent the threadedinserts from rotating about the insert axis.
 11. The leadscrewmechanical drive of claim 10, wherein the preload structure comprises afirst-leadscrew follower pivot about which the first leadscrew followerpivots, a second-leadscrew follower pivot about which the secondleadscrew follower pivots, wherein the first-leadscrew follower pivotand the second-leadscrew follower pivot are adjacent to each other, andthe preload structure that forces the two leadscrew followers to pivotaway from each other about their respective pivots.
 12. The leadscrewmechanical drive of claim 10, wherein the leadscrew thread pitch isgreater than the insert thread pitch.
 13. The leadscrew mechanical driveof claim 10, wherein the leadscrew thread pitch is less than the insertthread pitch.
 14. The leadscrew mechanical drive of claim 10, whereinthe leadscrew comprises a wire-wound leadscrew.
 15. The leadscrewmechanical drive of claim 10, wherein the insert thread is engaged tothe leadscrew thread over a circumferential distance of less than about10 degrees.
 16. The leadscrew mechanical drive of claim 10, wherein thepreload structure comprises a preload spring.
 17. A leadscrew mechanicaldrive comprising: a leadscrew having a leadscrew axis and a leadscrewthread with a nonzero leadscrew thread pitch, wherein the leadscrew isrotationally driven at a rate of turning about the leadscrew axis; and aleadscrew follower structure engaged to the leadscrew, wherein a rate ofadvance of the leadscrew follower structure relative to the rate ofturning of the leadscrew about the leadscrew axis is selectivelycontrollable between two different rates of advance.
 18. The leadscrewmechanical drive of claim 17, wherein the leadscrew follower structurecomprises a brake.